Histamine release from mast cells and blood basophils is being studied as one of the immunological mechanisms involved in inflammation. Among the histamine releasing agents employed are IgE antibody, environmental antigens, and complement factors (C5a and C3a) derived from human serum. The relationships between EgE crosslinking, basophil desensitization and histamine release were analyzed by kinetic studies. Release of inflammatory mediators from cultured mastocytoma cells is being compared with mechanisms of release from mast cells and basophils.